A Tower Mounted Amplifier (TMA) is used together with a Radio Base Station (RBS) and together they form, in the context of present description, a base station subsystem. A TMA comprises a Low Noise Amplifier (LNA) mounted as close as practical to one or several antennas of the base station subsystem. A TMA reduces the base transceiver station noise figure and therefore improve its overall sensitivity, in other words the mobile mast is able to receive weak signals.
The TMA used in Frequency Division Duplexing (FDD) based system is usually equipped with a duplexer who separates the transmission signals (TX) and the reception (RX) signals internally. The duplexer provides enough attenuation from transmission to reception in order to protect the LNA.
When the TMA is used in Time Division Duplexing (TDD) based system a circulator is usually provided to isolate the reception path from transmission path. This is necessary since a TDD-based communication system uses the same frequency for both downlink (corresponding to transmission) and uplink (corresponding to reception). However, the circulator cannot itself provide enough protection to the LNA, due to leakage in the circulator. The leakage in the circulator means that some power in the transmission path leaks into the reception path. This leakage may damage the LNA which is located in the reception path. In order to protect the LNA, a switch or a limiter may be placed between the circulator and the LNA in order to protect the LNA in case of high leakage in the circulator during a transmission period.
However, such a solution is associated with some drawbacks. In case a limiter is used to protect the LNA, the loop gain will increase the output inband ripple. In case a switch is used, the switch needs to be synchronized with the switch between downlink and uplink transmissions. In order to be able to synchronize the switch with the RBS, additional signal cable may used. However, it is expensive and it increases operating expenditures (OPEX) of the operator.
Such a solution, disclosed in the published international PCT application number WO2001058181A2, requires the current RBS platform to be modified and it will also increase the cost. Another issue with such a solution is reliability of the Bluetooth interface. There may be difficulties in case the frequency bands that the RBS and Bluetooth Interface use are close and/or are disturbing each other.